stateful-mtl-1.0.7: Typeclass instances for monad transformer stacks with an ST thread at the bottom.

Control.Monad.ST.Class

Description

This module contains the MonadST type class, which encapsulates a monad capable of lifting an ST computation. This type class is only intended to be implemented by the ST monad and any stack of monad transformers over an ST monad.

Presence of a MonadST instance implies that

The monad is single-threaded: performing an ST computation will not cause loss of referential transparency, and only one copy of its state thread will be available at any time.

Monad transformers can demand an underlying MonadST instance and use its state thread for their own safe computation in the ST monad.

Note: Most monad type classes cannot pass instances up through any instance of MonadTrans, because a transformer farther out may wish to override the inner instance. However, in MonadST we very specifically only want one state thread for any stack of transformers, and specifically one at the very bottom level. This justifies the very general MonadST propagation instance, (MonadST m, MonadTrans t, Monad m, Monad (t m)) => MonadST (t m) (not shown in Haddock for unknown reasons).